Clendon joined the Green Party in 1990. In both the 1999 and 2005 elections, Clendon polled third in the seat of Waitakere, ranked 19th and 12th on the party list, respectively. Clendon was the co-convenor of the Green Party from 2001 to 2004. He did not contest the 2002 general election because the party's constitution bars co-convenors from standing for parliament. Along with MPs Russel Norman and Nándor Tánczos, and former MP Mike Ward, Clendon contested the Green's male leadership role in 2005 after the unexpected death of co-leader Rod Donald, saying that it made sense to "appoint an out-of-Parliament leader, rather than stretch the sitting MPs even further." Russel Norman won the leadership after a vote at a party AGM in June 2006.
In the 2008 general election Clendon was ranked tenth on the Green list and stood as a candidate in the Helensville electorate, coming third with 5.96% of the electorate vote. Following the resignation of list MP Sue Bradford, Clendon entered Parliament as he was next on the Green party list. He became a Member of Parliament on 2 November 2009 and delivered his maiden speech to Parliament on 17 November. A private member's bill in Clendon's name was drawn from the ballot in February 2010. The Smart Meters Bill would require that domestic power users be advised on the options available for the use of smart meters in their homes. It was voted down by the Government later that year. In the, Clendon unsuccessfully contested the electorate but was re-elected as a list MP ranked eighth. In the, he stood in the electorate and was re-elected as a list MP ranked eleventh. He did not contest the 2015 Northland by-election, but was planning to stand in the seat again in the. On 7 August 2017, Clendon and fellow Green Party MP Kennedy Graham announced that they were planning to resign as Green Party candidates for the 2017 election, after revelations that Party co-leader Metiria Turei committed benefit and electoral fraud. Graham and Clendon stated that their resignations were due to the public positions she had taken regarding her offending, and her subsequent refusal to step down from her leadership role. The next day, both Clendon and Graham resigned from the Party caucus, after there were moves to remove them involuntarily. On 9 August 2017, Turei resigned as Co-Leader of the Party and as a list candidate for the 2017 election.
